compile multiple java files

Hi
I have another problem. I have a small project with 4 classes. I will compile this with:
gcj --main=MultiThreadTest -I/home/fabio/eclipse-workspace/workspace-gcj/GCJ_Threading_Test/src/ch/ntb/inf/fabio/gcj/ MultiThreadTest.java ThreadPool.java Calcula.java Logger.java -o tada

It gives me this error:
 /tmp/ccUNvvnJ.o: In function `main':
ccbV7AOD.i:(.text+0x24): undefined reference to `MultiThreadTest::class$'

What does this error means?
Thank you
Fabio Bucher